1. Field
The present disclosure relates to an apparatus and method for pumping optical fiber. More particularly, the present disclosure describes the use of a gradient refractive index (GRIN) structure on the side surface of an optical fiber to couple optical pump energy into the fiber for use in fiber lasers and amplifiers.
2. Description of Related Art
Numerous applications require the generation or amplification of optical signals. Optical systems using optical fiber are often preferred due to the cost, size, weight and other aspects of the optical fiber. Fiber optic systems are used in a large variety of commercial and military applications, such as in telecommunication systems, satellite communication systems, and radar systems. However, such applications and other applications known in the art may require the generation and/or amplification of optical signals with significant optical power in optical fiber.
To achieve high power levels, fiber amplifiers and fiber lasers require optical energy at high levels to be injected within the region of an optical fiber that includes an active gain medium that provides the optical gain. The active gain medium typically comprises the core of the optical fiber doped with rare earth elements such as erbium (Er), ytterbium (Yb), erbium-ytterbium (ErYb), neodymium (Nd), thulium (Tm), etc. When subjected to optical pump energy (typically having a wavelength from 800 nm to 1400 nm depending on the gain medium), the ions within the gain medium are excited to their upper lasing level, which provides that the optical fiber may then be used to generate or amplify optical signals.
One approach known in the art for pumping an optical fiber is by end-pumping. In end-pumping, the output from single-mode laser diodes may be directly coupled into the core of the fiber at the end of the fiber. A wavelength division multiplexer may be used to direct the pumping radiation from the laser diodes into the portion of the fiber containing the active gain medium. See, for example, “Optical fiber amplifiers: materials, devices, and applications,” S. Sudo, ed. Artech House Inc., 1999, pp. 406-407 and p. 432. However, this method of pumping provides relatively low power levels, since only a relatively low power level can be applied to the very small end surface of the fiber. Attempts to provide higher levels of power will generally result in laser-induced damage to the fiber. Typically, the maximum power output seen with end-pumping is about 100 mW, since 100-200 mW is typically the maximum power level that can be coupled into the fiber core at the lowest transverse mode from laser diodes.
Higher output powers may be obtained by side-pumping optical fibers that are dual clad fibers. A dual clad fiber typically comprises a doped single-mode core surrounded by a multi-mode inner cladding that guides pump radiation along and around the doped core. Optical pump energy may then be coupled into the inner cladding from the side of the optical fiber using various techniques known in the art. Dual clad optical fibers provide the ability to couple more power into the cladding, thus resulting in a much higher output power than that typically obtained from a single mode fiber. The dual clad optical fiber also has a cross-section that is much larger than the cross-section of single-mode fiber, which also provides for an increased pump power and, therefore an increased power output. However, the power output of side-pumped optical fibers using techniques known in the art may still be rather small when compared to the power output provided by other laser technologies.
One technique for side-pumping an optical fiber is described in U.S. Pat. No. 5,854,865, by Goldberg and issued Dec. 29, 1998. Goldberg describes fabricating a groove or micro-prism into at least a portion of the inner cladding of an optical fiber. Pump energy is then applied to the optical fiber from a direction opposite the groove or micro-prism such that the pump light travels through the fiber in a direction generally perpendicular to the core. The pump light then reflects from the facets of the groove or micro-prism and is directed along the core of the fiber. However, cutting a groove or micro-prism in the inner cladding may impair the mechanical property of the fiber resulting in decreased reliability for the fiber. In addition, the groove or micro-prism provides only a relatively small surface for reflection of the pump light, thus restricting the total energy of the pump light to avoid optical damage to the surface of the groove or micro-prism.
Another approach for side-pumping is disclosed in Heflinger, et al., “Apparatus for Optically Pumping an Optical Fiber from the Side,” EP 1 065 764 A2, published Jan. 3, 2001. Heflinger, et al. describe an optical fiber using a diffraction grating to receive and reflect pump light into the inner cladding of a dual clad active optical fiber. According to Heflinger, et al., the diffraction grating may be imprinted on the surface and into the inner cladding, may be separately disposed subjacent or adjacent the surface of the inner cladding, or may be provided by recording a diffraction grating within the inner cladding. However, imprinting a grating on and/or within the inner cladding may weaken the fiber mechanically. Providing a diffraction grating subjacent or adjacent the inner cladding may complicate the fabrication of the pumped optical fiber. Recording a diffraction grating in the inner cladding may comprise forming an inclined Bragg grating in the inner cladding. However, because the refractive index variation Δn typically available in active optical fibers is low (usually, Δn<10−3), the grating must be rather thick (>100 μm) for high diffraction efficiency. In turn, the pump beam should have very low divergence (<3×10−4 rad) to satisfy the Bragg conditions, something that may be complicated to realize in practice.
Still another approach for side-pumping an optical fiber is presented in Th. Weber, E. Luthy, H. P. Weber, “Side-pumped Fiber Laser,” Apply. Phys. B., v. 63, pp. 131-134, 1996. Weber, et al. propose the use of a prism connected to the optical fiber through immersion oil. Both the prism and the immersion oil have the same refractive index as the fiber cladding. The pump energy is directed into the cladding at a large angle of incidence through the refracting side of the prism. The coupling efficiency for such a system was reported as 45.3%. However, to increase the interaction area and, therefore, to increase the total power coupled into the fiber without laser-induced damage of the fiber surface, Weber et al. disclose that the pump energy is applied at an angle of incidence close to 90°. This angle of incidence results in having a small surface area available on the refracting side surface of the prism. Hence, the refracting surface may be damaged with elevated levels of pump energy.
Still another approach for side-pumping is disclosed in Manzur, “Side-pumped Fiber Laser,” WO 00/54377, published Sep. 14, 2000. Manzur discloses creating a coupling window integrally formed within a channel formed in an upper side of the fiber cladding of a single clad optical fiber. This structure then provides for direct pumping of the core of the single clad fiber. Manzur discloses that different shapes for the windows may be used as well as different materials for the windows to improve the coupling efficiency. Manzur discloses that the window may comprise a graded index material or step index material to improve the design of the side-pumping device and to increase the coupling area. However, the small diameter of the core of the optical fiber serves to limit the size of the laser beam applied to the coupling area. This then limits the optical power that may be used in pumping the optical fiber.
Still another approach for side-pumping is disclosed in Hollister et al., “Fiber amplifier having a prism for efficient coupling of pump energy,” U.S. Pat. No. 6,625,354, Holister teaches an optical fiber amplifier employing a prism to couple pump energy into the pump core of a dual clad fiber. Holister shows embodiments using traditional optical lens in the optical path between the pump source and the fiber being pumped, but also shows an embodiment wherein a traditional optical lens is replaced with graded index lens which has a varying index of refraction across its optical axis.
As noted above, prior art approaches may limit the amount of pumping energy that may be applied to an active optical fiber, thus limiting the magnitude of the optical output from the active optical fiber. High-power laser diodes are available for use in applying pump energy to optical fibers, but the prior art approaches may limit the amount of power that can be applied from these laser diodes and other pump energy sources because of concerns about laser-induced damage to the pumped fiber. Therefore, there is a need in the art for an apparatus and method that provides for increased pumping energy to be applied to an active optical fiber without resulting in damage to the pumped fiber.